I watched this last night. Very good art horror. I can't really comment too much as it unravels as it goes. My wife did not like this movie but I was fully enjoying this. It was almost mesmerizing and the score is in that theremin (whhrring) sound that you hear in old movies usually sci-fi.
I liked it a lot except for one thing (spoilers): two actresses who are not physically similar enough for it be intuitive they the same person at two different time periods, and that makes the ending not have the intended kick. I simply assumed when the demon was exorcised from Kat (Kiernan Shipka) it hung around and eventually possessed a different inmate named Joan (Emma Roberts) and that when Joan screamed at the end that was because the demon un-possessed her allowing her to realize what she had done... which is a horrific ending. But that was not at all what the director had in mind. I had to look up the explanation online, and find out the two actresses were playing the same character, and that the reason that Kat -- who had assumed the identity of someone named Joan to escape the hospital years later -- screamed at the end was due to her inability to call back the demon because she actually wanted to be possessed. I'd call that a major flaw, unless I'm totally wrong and this is more intuitive for everybody else. But other than that, I love movies where the director is not afraid to have periods of no dialogue allowing the soundtrack and actors' expressions and body language do the work. The lesson to be learned here is about directing in a way to slowly build up atmosphere and tension and even some confusion which too few horror movies do nearly as well. The creepiest scene was when Rose saw Kat bowing rapidly before the furnace. What did not work for me at all was showing the shadow of the demon -- I kept thinking of a giant rabbit.